1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a method and apparatus for inspecting an electrical defectiveness of an LCD by using an electrostatic discharge damage (ESD) protection device.
2. Description of the Related Art
Display apparatuses have become important as visual information transferring media. Among the display apparatuses, a cathode ray tube is widely used at present, but is disadvantageous in that its weight and volume are large. Therefore, various types of flat display apparatuses have been developed that are capable of overcoming the defects of the cathode ray tube. An LCD, a field emission display (FED), a plasma display panel (PDP), and an electroluminescence (EL) display are different examples of flat display apparatus. Most of these apparatuses are available in the market.
The LCD device is easily adaptive due its smallness which improves productivity. Thus, it is quickly replacing the cathode ray tubes in many applications. In particular, the LCD device of an active matrix type for driving a liquid crystal cell by using a thin film transistor (hereinafter referred to as “TFT”) has an advantage in that the picture quality it provides is excellent, and its power consumption is low. Such LCDs have been rapidly developed into a large size and high definition due to the recent productivity technology and research.
A process for fabricating the LCD device of the active matrix type is divided into a substrate cleaning, a substrate patterning, an alignment forming/rubbing, a substrate assembling/a liquid crystal material injecting, a mounting, an inspecting and a repairing, etc.
Generally, impurities on the substrate surface of the LCD device are removed by a detergent in the substrate cleaning process.
The substrate patterning process is divided into a patterning of an upper substrate (color filter substrate) of an LCD and a patterning of a lower substrate (TFT array substrate) of the LCD. There are formed a color filter, a common electrode, a black matrix, etc. on the upper substrate. There are formed signal wirings such as data lines and gate lines on the lower substrate, TFTs (thin film transistors) each at an intersection of the corresponding data line and the corresponding gate line, and pixel electrodes each in a pixel region between the corresponding gate line and the corresponding data line connected to a source electrode of the TFT.
An alignment film is applied to each of the upper substrate and the lower substrate in the alignment film forming/rubbing process and the alignment film is rubbed by a rubbing material.
In the substrate assembling/the liquid crystal injection process, the upper substrate and the lower substrate are bonded together by using a sealant, and the liquid crystal material and spacers are injected through a liquid crystal injection hole. Then the liquid crystal injection hole is sealed.
In the mounting process of the liquid crystal panel, a tape carrier package (TCP) mounted with integrated circuits such as a gate drive integrated circuit and a data drive integrated circuit, is connected to a pad part on the substrate. Such drive integrated circuits may be directly mounted on the substrate by using a chip on glass (COG) method other than a tape automated bonding (TCB) using the TCP described above.
The inspecting process includes a first electrical inspection performed after a variety of signal wirings and the pixel electrodes are formed and a second electrical inspection and a visual inspection performed after the substrate assembly/liquid crystal injection process. Specifically, the electrical inspection of the signal wirings and the pixel electrodes of the lower substrate followed by the substrate assembling may reduce an undesirable ratio and a waste matter and may find a defective substrate capable of repairing at an early stage.
The repairing process performs a restoration for a repairable substrate determined by the inspecting process. However, in the inspecting process, defective substrates beyond repair are discarded.
The electrical inspection of the lower substrate (TFT array substrate) of a general LCD, which is performed before the substrate assembling, frequently employs a method using an apparatus shown in FIG. 1. Referring to FIG. 1, this electrical inspection process includes: placing a modulator 10 over a TFT array substrate 11 of an LCD to be tested with a designated gap, applying a test voltage (Vtest) to the modulator 10 while maintaining the gap, detecting a light reflected from the modulator 10 in response to the test voltage, and determining an electrical defectiveness of signal wirings 17 and 18 (data and gate lines) of the LCD substrate.
In the modulator 10, a polymer-dispersed liquid crystal (hereinafter referred to as “PDLC”) 14 is put between an upper transparent substrate 12 having a common electrode 13 formed thereon and a lower transparent substrate 15. In the modulator 10, a reflection sheet 16 is mounted on a rear surface of the lower transparent substrate 15. The modulator 10 has an air nozzle and a vacuum nozzle for auto-gapping that is used to maintain a designated gap between the modular 10 and the TFT array substrate 11 being inspected.
Above the modulator 10, a lens 21 is provided for focusing a light 22 from a light source (not shown) onto the modulator 10 and for transmitting any light 22 reflected from the modulator 10 during the inspection.
The TFT array substrate 11 being tested includes thereon TFTs 19, the signal wirings 17 and 18 (data and gate lines crossing each other in a matrix format) and pixel electrodes 20. The TFT array substrate 11 is formed in a liquid crystal display apparatus of the active matrix type.
The electrical inspection of the TFT array substrate 11 begins by loading the substrate 11 to be tested below the modulator 10 and lowering the modulator 10 with a certain gap maintained between the modulator 10 and the substrate by auto-gapping. While the gap between the modulator 10 and the substrate 11 is maintained at a predetermined effective gap, the light 22 from the light source is radiated towards the modulator 10 and focused onto the modulator 10 via the focusing lens 21, and simultaneously a test voltage (Vtest) is applied to the common electrode 13. And a test data applied from a driving circuit in a jig (not shown) is applied to the data lines 17 and a test scan signal is applied to the gate lines 18. Then, an effective electric field is applied to the PDLC 14 between the common electrode 13 of the modulator 10 and the pixel electrode 20 to be tested.
If the electric field is not applied, the PDLC 14 causes the light 22 from the light source above the modulator 10 to be scattered. However, if the effective electric field (E) is applied, the liquid crystal molecules in the PDLC 14 become aligned to the direction of the effective electric field (E) and cause the light from the light source to be transmitted through the PDLC 14. That is, if the wirings 17 and 18 properly work, then the PDLC 14 will cause the light from the light source to be transmitted through the PDLC 14. Accordingly, during this electrical inspection process, the liquid crystal layer of the PDLC 14 corresponding to the pixel electrode 20 to which the voltage is properly applied and transmitted, causes the light 22 to be transmitted through the PDLC 14. However, if the voltage is not properly transmitted to the pixel electrode 20, e.g., due to a defect in the wiring(s) associated with the pixel electrode 20, then the liquid crystal layer of the PDLC 14 causes the light 20 to be scattered in that part.
The light 22 transmitted through the liquid crystal layer of the PDLC 14 is reflected on the reflection sheet 16 of the modulator 10 and is reversely directed back to the lens 21, while the light 22 scattered in the liquid crystal layer of the PDLC 14 is vanished and is not incident to the reflection sheet 16. The light reflected by the reflection sheet 16 of the modulator 10 and transmitted out from the modulator 10 is then received by a charge-coupled device (CCD) (not shown) via the focusing lens 21. The reflected light is then converted into an electrical signal and transferred to a display apparatus via a signal processing circuit. A testing inspector monitors an image or data displayed on the display apparatus to determine whether or not there is a defect in the wirings 17 and 18 of the substrate 11 and performs a second, closer inspection about the signal wirings 17 and 18 of doubtable point if a defect is initially detected.
The modulator 10 can provide reliability, but has a defect of high price. Further, since the inspection region is narrow as compared with the full area of the substrate 11, the modulator 10 must repeat the process of inspection for each of different wirings sequentially by moving a designated distance in the vertical or horizontal direction and then stopping temporarily for auto-gapping. This requires a significant amount of inspection time.